Additives to stimulate microbial activity
The contents of the additives in nutrients and readily available forms of carbon influence the microbial activity. For example, adding jaggery increased the number of microorganisms and thus enhanced the enzymatic degradation of cellulose during composting of green wastes (Gabhane et al, 2012). Similar results were observed with the addition of fish pond sediment, spent mushroom substrate and biochar (Zhang and Sun, 2014; 2017). The effect of biochar on the microbial activity was probably due to their effect on microbial habitat and protection from grazers (Meng et al, 2013; Wei et al, 2014). Alkaline substrates such as fly ash and lime are also additives with high contents in nutrients, however they do not favour microbial activity due to their high pH (Fang et al., 1999). At high rates, fly ash addition inhibited phosphatase, B-glucosidase or dehydrogenase but did not affect urease activity, indicating that some stages of N-compound degradation were unchanged (Wong et al, 1997).
Additives to improve aeration
An optimal aeration during composting is required as explained by Gao et al (2010). A low aeration rate might lead to anaerobic conditions while a high aeration rate might result in excessive cooling and thus preventing thermophilic conditions. The common way to enhance the aeration and thus favour the microbial activity during composting is through mechanically turning the composted material (Chen et al, 2015; Chowdhury et al, 2014; Manu et al, 2017) and forced aeration through pipes (Ogunwande and Osunade, 2011). These actions are not necessary in the case of vermicomposting due to worm activity. The costs of the pile turning or forced aeration may be reduced by the use of bulking agents, such as biochar, residual straws, wood chip or sawdust, crushed branches, etc. since they enhance the natural aeration and porosity of the composting pile (Kulcu and Yaldiz, 2007; Czekala et al, 2016). Due to their numerous pores and low moisture content, these bulking agents support the formation of inter and intraparticles voids (Iqbal et al, 2010). Moreover, according to De Bertoldi et al (1983), the biological oxidation rate is directly related to the surface area exposed to the microbial attack.
Additives to regulate the moisture content
The moisture content during composting influences the oxygen uptake rate and thus, the microbial activity and the degradation rate. The optimal water content for organic matter biodegradation has been established within 50-70% (Richard et al, 2002). However, some organic wastes have a higher or lower moisture content; for example, sewage sludge, which has a moisture content ranging from 80% to 90%. Such high humidity might favour odour production during composting (Jolanun et al, 2008). The bulking agents commonly chosen to offset the high moisture content of these organic wastes are fibrous materials with low moisture content (Miner et al., 2001, Eftoda and McCartney, 2004, Doublet et al., 2011), which can absorb part of the leachate. The leachate absorption by cornstalk, sawdust or spent mushroom substrate was also observed by Yang et al (2013) during kitchen waste composting. Furthermore, Chang and Chen (2010) showed that increasing water absorption capacity by increasing sawdust addition during food waste composting also resulted in higher degradation rate due to more air flow through the particles.
On the contrary, water loss during the first days of composting might delay the composting process and necessitates water sprinkling. The addition of an additive with a capacity of water retention, such as clays, might limit water losses. Li et al (2012) showed that the initial moisture decrease was buffered by the presence of bentonite, due to its swelling performance. During composting of green wastes, the water holding capacity was also increased by the addition of ash (Belyaeva and Haynes, 2009), or phosphate rock (Zhang and Sun, 2017). However, some additives such as egg-shells had no influence on water holding capacity (Soares et al, 2017), and may even have a negative effect on the biological activity.
Additives to buffer pH
The pH varies during composting with a decrease during the early stages of composting and an increase during the later stages (Onwosi et al, 2017), which affects the microbial activities. Some additives are used to increase the pH and thus enhance the composting of acid feedstocks, such as food waste (Wong et al, 2009). The addition of an inoculum consortium was reported to result in a pH increase from 4.3 to 6.3 during food waste composting (Manu et al 2017). This may be explained by the degradation of acids along with organic matter through enhanced biological activity. The use of bulking agents such as bagasse, paper, peanut shell and sawdust might also increase the pH during composting (Iqbal et al 2010). Fly ash or lime amendments also increased the initial pH of the co-compost (Fang and Wong, 1999; Gabhane et al, 2012), but such alkaline amendments might inhibit the metabolic activity. Wong et al (1997) observed less thermophilic bacteria in the initial phase when sludge was co-composted with 25% fly ash, demonstrating the adverse effect of alkaline amendments on bacteria.
The increase of pH during the thermophilic phase was offset by using bamboo charcoal, that adsorbed ammonia, and inhibited OH- release (Chen et al, 2010). Similar results were observed by Venglosky et al (2005) with the addition of 2% zeolite to pig slurry composting. A lower pH may also decrease nitrogen loss, by avoiding ammonia volatilization, which occurs at high pH Chen et al (2010). Finally, the addition of elemental sulphur considerably decreased the pH during poultry manure composting (Mahimairaja et al, 1994), mainly explained by the oxidation of elemental sulphur producing H2SO4, and thus increasing the H+ ion concentration.
Influence of the additives on gas emissions during composting
NH3 and sulphur are the main odorous gases emitted during composting. No reference has been found in the literature on vermicomposting and odour gas emissions, likely due to the fact that worm activities allow a continuous aeration of the composting pile, which limits formation of anaerobic zones responsible for odour emissions. By improving oxygen transfer inside the composting pile of municipal wastes with the addition of rice straw (1:5), Shao et al (2014) decreased the cumulative malodorous sulphur-containing gases. Koivula et al (2004) obtained similar results with ash addition and Steiner et al (2010), Hua et al (2009) and Khan et al (2014) with biochar addition. Another way to reduce the odour emissions is to trap the nitrogen excess and thus reduce the ammonia contents in the composting pile. The addition of natural zeolite as an adsorbent appears to reduce ammonia odours (Turan, 2008), Wang et al (2014), Lefcourt and Meisinger (2001), with increasing effect on odour reduction when more zeolite is added. Mahimairaja et al (1994) confirmed that carbonaceous materials with high cation exchange capacity reduced the cumulative loss of NH3 due to the immobilization of NH4+ and enhanced this process by acidifying compost with elemental sulphur. Bernal et al (1993) and Witter and Lopez-Real (1988) showed that zeolite placed on the top of the compost pile reduces ammonia emissions. Zang et al (2017) observed that chemical additives (a nitrogen electron acceptor, sodium nitrate and sodium nitrite) efficiently controlled sulphur odours, by reducing the emissions of dimethyl sulphide and dimethyl disulphide by 92.3% and 82.3% respectively, without altering the composting process. Similarly, Yuan et al (2015) efficiently reduced H2S emissions with a chemical product (FeCl3). Ren et al (2010) and Jeong and Hwang (2005) observed a reduction of the ammonia loss during pig manure composting due to a chemical reaction (struvite precipitation) with an absorbent mixture of magnesium hydroxide and phosphoric acid. By contrast, sawdust, cornstalk and spent mushroom substrates did not influence ammonia emissions (Yang et al, 2013; Yuan et al, 2015), while bentonite even increased them during swine manure composting (Jiang et al, 2014).
The amounts of GHG released during composting of different initial feedstock with different additives are shown in Table 2. GHG emissions from co-composting processes have been largely studied but few data are available on vermicomposting processes and let alone on co-vermicomposting. Moreover, among GHG gases, only CH4 and N2O emissions during composting are accounted for in national GHG inventories, but CO2 of biogenic origin is not (IPCC 2014).
N2O can be produced under aerobic conditions, during incomplete nitrification/denitrification and anaerobic conditions, when a lack of O2 leads to nitrate accumulation. Additives affect differently the N2O emissions according to the initial feedstock. For instance, adding mineral as phosphogypsum (Hao et al, 2005; Luo et al, 2013) significantly reduced N2O emissions during manure composting, probably by increasing SO42- content of the compost or affecting the nitrification process while there is no effect of the bulking agent (woodship and polyethylene tube) on N2O emissions during municipal wastes composting (Maulini-Duran et al, 2014). Finally, during kitchen waste composting, addition of sawdust significantly reduced N2O emissions (Yang et al, 2013).
In a composting pile, CH4 emissions generally occurred due to a lack of oxygen, an excessive moisture or the presence of anaerobic zones (Amlinger et al, 2008). Contrary to N2O emissions, CH4 emissions appear to be more dependent on the properties of the additives than those of the initial feedstock. Thus, to reduce CH4 emissions, two types of additive might be applied, affecting directly or indirectly the carbon cycle: (1) inoculation of methanotrophic bacterium (Luo et al, 2014) might enhance CH4 oxidation and thus reduce CH4 emissions; (2) addition of bulking agents might decrease anaerobic zones. For instance, recent studies (Yang et al, 2013; Maulini-Duran et al, 2014) observed that addition of bulking agents as cornstalk, sawdust, spent mushroom substrate or even polyethylene tube improved the compost structure, decreased the moisture content and thus the CH4 emissions. Moreover, cornstalk appeared more efficient than sawdust and spent mushroom substrate due to its larger particle size (Yang et al, 2013). However, some organic bulking agents such as woodchip (Maulini-Duran et al, 2014) had the opposite effect, generating even more CH4 than a regular compost. The authors explained these results by a rapid consumption of easily biodegradable matter, leading to the formation of compact zones.
Compared to regular composting, CO2 emissions are generally increased by the addition of organic materials with readily decomposable carbon (paper, straw, peat materials, etc) (Mahimairaja et al, 1994), and decreased by the addition of organic materials rich in lignin or other slowly degrading organic compounds (Mahimairaja et al, 1994; Chowdhury et al, 2014). Biochar addition during composting has contradictory effects on CO2 emissions compared to regular compost, either increasing (up to 8% in Czekala et al 2016), or decreasing them (Chowdhury et al 2014). Such contradictory effects were also observed for biochar addition during vermicomposting (Barthod et al, 2016). Addition of other relatively inert materials, such as plastic or pumice increased CO2 emissions, through aeration improvement and microbial activity enhancement (Czekala et al, 2016; Wu et al, 2015). Finally, reducing CO2 emissions from composting without altering the biodegradation process appears difficult. A possible method to limit CO2 emissions is to trap emitted CO2 with an adsorbent, such as red mud or to protect carbon from decomposition by associating it with clays or amorphous hydroxyl-Al (Bolan et al, 2012; Barthod et al, 2016; Haynes and Zhou, 2015).
Effects of application of co-compost on soil properties and plant development
General soil definition and soil “health” definitions
Soil can be defined as the fine layer (from some cm to 3-4 m) which is at the interface between the atmosphere and the lithosphere. Soil is difficult to describe as it the assemblage of micro and macroaggregates, composed of water, air spaces, organic matter and minerals. These aggregates are mainly formed by microbial activity or micro and macrofauna (Bossuyt et al, 2005). Mineral compounds originate from the rock alteration while organic matter comes from vegetal or animal organisms (exudates or decomposition). Thus, the organic matter in soil is complex as it comprises different compounds, under various forms. Soils are classified according to their structure and mineral composition. The mineral composition is generally linked to the climate and localisation (Fig. 5). Therefore, there is not one soil but several different soil types.
Soil health is defined as “the capacity of a specific kind of soil to function, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation” (Doran and Zeiss, 2000). Therefore, soil is at the heart of numerous ecosystem services and a “good” soil health is necessary to fulfil these services as shown by Fig.6.
Table of contents :
Chapter 1 Improving the environmental footprint and properties of compost and vermicompost through mineral, organic and biological additives.
1. Principle and differences of two waste management strategies: composting and vermicomposting
1.1. Definition of the composting process
1.2. Definition of the vermicomposting process
1.3. Advantages and disadvantages of these processes
2. Optimizing composting processes by using additives
2.1. Different types and sources of additives
2.2. Effects of additives on the composting processes
2.2.1. Effects of additives on composting temperature profile
2.2.2 Additives to stimulate microbial activity
2.2.3. Additives to improve aeration
2.2.4. Additives to regulate the moisture content
2.2.5. Additives to buffer pH
4. Influence of the additives on gas emissions during composting
4.1 Odour emissions
4.2. GHG emissions
3. Effects of application of co-compost on soil properties and plant development
3.1. General soil definition and soil “health” definitions
3.2. Qualities of co-compost as potting media or soil amendments: influences of additives on nutrient availability, metal availability and soil fertility.
3.2.1. Reduction of environmental hazards due to heavy metals
3.2.2. Influence of additives on the nutrient contents and availability
3.2.3 Co-compost stability and carbon sequestration potential of soil amended with cocompost
4. Stabilization of organic matter: mechanisms occurring in soil Innovative waste treatment by composting with minerals and worms
4.1. Physical and chemical stabilization mechanisms
4.1.1. Biochemical protection through organo-mineral associations
4.1.2. Physical protection through macro- and microaggregates
4.1.3. Chemical protection through recalcitrance
4.2. Worm implications in stabilization of organic matter
Chapter 2 Influence of worms and minerals on fresh organic matter degradation and compost stabilization
1. Materials and methods
2.1. Fresh organic matter and mineral properties
2.2. Worm species
2.3. Model (compost and vermicompost) systems
2.4. Analysis of mineral and organic constituents
2.5. NMR spectroscopy
2.6. Transmission electron microscopy
2.7. Calculation and statistical analysis
2.1. Macroscopic observations of organic residue degradation
2.2. Microscopic structure of the end-products: TEM analysis
2.3. Carbon dynamics during incubations
2.4. Properties of the final products after 196 days of incubation
2.4.1. Carbon content and dissolved organic carbon
2.4.2. NMR spectroscopy
3.1. Carbon mineralisation without worms: effect of clay and iron oxide addition
3.2. Changes in compost chemical composition with mineral addition
3.3. Influence of worms on the protection of fresh organic matter through organo-mineral associations
Chapter 3 Addition of clay, iron oxide and worms during composting affects N, P nutrients, microbes and physicochemical properties of composts.
2. Materials and Methods
2.1. Experimental set-up
2.2. Analytical methods
2.2.1. Elemental analysis
2.2.2. Phosphorus fractionation
2.2.3. Microbial biomass
2.2.4. Phospholipids fatty acids (PLFA)
2.3. Statistical analysis
3.1. Physico-chemical characteristics of the composts and vermicomposts
3.2. Microbial abundance and community structure in composts and vermicomposts
3.3. Phosphorus and nitrogen availability
4.1. Influence of minerals on the compost physico-chemical characteristics
4.2. Influence of minerals on the microbial parameters of the composts
4.3. Influence of minerals on the compost nutrient availability
4.4. Worms may counterbalance the negative effects of minerals
Chapter 4 Influence of application to soil of co-compost and co-vermicompost produced with clay minerals and iron oxides on carbon mineralization and distribution
2. Materials and Methods
2.1. Soil sampling and preparation
2.2. Composts and vermicomposts materials
2.3. Experimental design
2.4. Microbial biomass
2.5. Carbon fractionation
2.6. Analytical methods
2.7. Statistical analysis
3.1. Characteristics of the organic amendments
3.2. Amended soil characteristics
3.3. Carbon mineralization and evolution of the microbial biomass
3.4. Carbon repartition in fractions of amended soils
4.1. Effect of co-compost application to soil on organic matter mineralization and microbial biomass carbon Innovative waste treatment by composting with minerals and worms
4.2. The application of co-compost changed soil carbon distribution and soil nitrogen forms
4.3. Co-vermicompost with minerals: an efficient strategy to increase soil carbon storage?
Chapter 5 Growth of Arabidopsis Thaliana in potting media made of co-vermicompost and co compost with clay minerals
2. Materials and methods
2.1. Potting media and organic amendments
2.2. Experimental set up
2.3. Plant and substrate analysis
2.3.1. Seed, root and shoot biomass
2.3.2. Measurement of leaf surface area and projected surfaces
2.3.3. Heavy metal contents and nutrient contents
3.1. Carbon and nitrogen composition of roots and shoots during plant growth
3.1.1. Non fertilized treatments
3.1.2. Fertilized treatments
3.2. Evolution of plant growth parameters
3.2.1. Non fertilized treatments
3.2.2. Fertilized treatments
3.3. Length of inflorescence and seed yields
4.1. Growth parameters and seed yields with compost and vermicompost
4.2. Influence of clay application within compost and vermicompost
4.3. Nitrogen concentration in plant tissues depends on the substrate and the presence of clay
Chapter 6 Addition of worms during composting with red mud and fly ash reduces CO2 emissions and increases plant available nutrient contents.
2. Materials and methods
2.1 Fresh organic matter, additives and worms
2.2 Elemental and chemical analysis
2.3 Incubation experiment and measurements of CO2, CH4 and N2O emissions
2.4 Microbial biomass and phospholipid fatty acid analysis (PLFA)
2.5 Phosphorus fractionation
2.6 Calculation and statistical analysis
3.1 End-product characteristics
3.2 Gas emissions and decomposition rate
3.3 Microbial biomass amounts and communities
3.4 Phosphorus fractions and nutrient contents
4.1 Alkaline materials influenced GHG emissions, in particular CO2
4.2 Alkaline materials influenced the qualities of the final products
4.3 Worms increased co-compost qualities but also GHG emissions